In all seriousness I do them as needed. Earlier this year I did my wife's Bernina for the first time in its life. It's a 930 made in the mid 80s. I've seen lint but this thing was packed inside. Wife was appalled at what was inside. Many of our machines haven't been used much in a while so they don't have anything in to need cleaned out. Others get used off and on and clean them as I go. Still others seem to get dirtier faster than the norm so they get cleaned every time their used.
But at over ...... um 30+ machines I really don't have the time to dig out all of them and clean the bobbin areas. Some of them are quite burried.
Joe